The present invention relates to an inrush current limiting circuit, a power source device and a power conversion device; in particular, relates to an improvement thereof to limit an inrush current with high reliability and durability while reducing the overall weight and volume of the device even under a high rated power.
FIG. 16 is a circuit diagram showing a structure of a conventional power conversion device (which will be hereinafter referred to as xe2x80x9cfirst prior artxe2x80x9d) having an inrush current limiting circuit. This power conversion device 451 comprises a main power source 10, an inrush current limiting circuit 301 and an inverter 13. The main power source 10 is connected to input terminals PP and NN of the inrush current limiting circuit 301, and the inverter 13 is connected to the output terminals P and N. The power conversion device 451 is utilized by connecting a three-phase load 100, such as a motor, to output terminals U, V and W of the inverter 13.
The inrush current limiting circuit 301 comprises switches 11 and 11a, a resistor 11b, and a capacitor 12. The main power source 10 supplies the input terminals PP and NN with a DC (i.e. direct current) source voltage VDC1. The capacitor 12 so functions as to hold the source voltage VDC1 at a constant level by means of its capacitance CDC. The switch 11, structured as a relay, functions as a main switch that turns ON when starting the inverter 13 and turns OFF when stopping it.
The switch 11a and the resistor 11b are interposed between the main power source 10 and the capacitor 12 so as to limit an inrush current which flows to charge the capacitor 12 immediately after the switch 11 turns on. After the switch 11 turns ON to start the inverter 13, the switch 11a remains OFF until charging of the capacitor 12 is almost completed. Accordingly, the inrush current is limited by the resistor 11b. 
When the charged voltage across the capacitor 12 almost reaches the source voltage VDC1, the switch 11a turns on. Thereafter, the inverter 13 starts its normal operation. Thus, a DC current is supplied to the inverter 13 by the main power source 10 with little loss, during the normal operation of the inverter 13.
However, since a large current supplied by the main power source 10 flows through the switch 11a and the resistor 11b, the switch 11a and the resistor 11b are required to have a high rated power, as well as the switch 11 as the main switch. In the power conversion device 451, therefore, it has been a problem that overall weight and volume of the device are large and a manufacturing cost thereof is high.
Further, since the switch 11 and the switch 11a (in particular, the switch 11) have to operate mechanically while a high voltage being applied, the reliability and durability thereof have been problematic. In particular, arcing is influential to the reliability and durability. Moreover, conduction losses of both the switch 11 and the switch 11a during the normal operation of the inverter 13 have also been not negligible concerns.
FIG. 17 is a circuit diagram showing a structure of another conventional power conversion device (which will be hereinafter referred to as xe2x80x9csecond prior artxe2x80x9d) having an inrush current limiting circuit. This power conversion device 452 is disclosed in Japanese Patent Application Laid-Open No. 8-289546 (1996), and is characteristically different from the power conversion devices 451 in that a transistor 30a and a base drive circuit lid are used for an inrush current limiting circuit 302 in place of the switch 11a. 
Even though the power conversion device 452 eliminates the problem arising from the switch 11a, it still needs the resistor 11b which must have a high rated power. Therefore, the power conversion device 452 has also been disadvantageous in that the weight and volume of the device are large and the manufacturing cost thereof is high. Further, the switch 11 has to operate mechanically while a high voltage being applied similarly to that of the power conversion device 451, and therefore, the reliability and durability of the switch 11 has been problematic.
FIG. 18 is a circuit diagram showing a structure of still another conventional power conversion device (which will be hereinafter referred to as xe2x80x9cthird prior artxe2x80x9d) having an inrush current limiting circuit. This power conversion device 453 is characteristically different from the power conversion device 451 according to the first prior art in that an NTC (negative coefficient thermistor) 11c is used for an inrush current limiting circuit 303 in place of the switch 11a and the resistor 11b. 
Usually when starting the inverter 13, since the NTC 11c is low in temperature, it has high resistance. The inrush current flowing after the switch 11 turns ON, therefore, is limited by a high resistance of the NTC 11c. The temperature of the NTC 11c rapidly rises up due to loss heat generated in the NTC 11c. Accordingly, the resistance of the NTC 1c rapidly decreases, which decreases loss in the NTC 11c. 
The loss generated in the NTC 11c during the operation of the inverter 13, however, is not negligible, and therefore, the power conversion device 453 has been disadvantageously applicable only to devices having a low rated power (e.g. few kW or less). Further, if the inverter 13 starts operation before the NTC 11c has sufficiently been cooled down, e.g. the inverter 5 restarts immediately after it has stopped, the NTC 11c does not sufficiently function as an inrush current limiter, which has also degraded the reliability of the device.
Accordingly, it is an object of the present invention to obtain an inrush current limiting circuit which eliminates the above mentioned problems and limits an inrush current with high reliability and durability while reducing overall weight and volume of the device even under a high rated power, and to provide a power source device and a power conversion device having the inrush current limiting circuit.
In order to achieve the object, a first aspect of the present invention is directed to an inrush current limiting circuit. The inrush current limiting circuit comprises: a first capacitor; a first switching element of a charge driven type, one main electrode thereof being connected to one end of the capacitor; and a second capacitor, one end thereof being connected to the one main electrode of the first switching element, and other end thereof being connected to a control electrode of the switching element.
According to a second aspect of the present invention, in the initial current limiting circuit of the first aspect, the inrush current limiting circuit further comprises: a first resistor interposed between the one main electrode and the control electrode of the first switching element and connected in series to the second capacitor.
According to a third aspect of the present invention, in the initial current limiting circuit of the first aspect, the inrush current limiting circuit further comprises: a first diode connected in inverse-parallel to the first switching element.
According to a fourth aspect of the present invention, in the initial current limiting circuit of the first aspect, the inrush current limiting circuit further comprises: a drive circuit connected to the control electrode of the first switching element, and driving the first switching elements in response to a control signal.
According to a fifth aspect of the present invention, in the initial current limiting circuit of the fourth aspect, the drive circuit comprises a first output resistor interposed into a path of a drive current which drives the first switching element to turn ON.
According to a sixth aspect of the present invention, in the initial current limiting circuit of the fourth aspect, the drive circuit comprises: a second switching element, one main electrode thereof being connected to a source line; a first output resistor, one end thereof being connected to the other main electrode of the second switching element, and other end thereof being connected to the control electrode of the first switching element; and a third switching element, one main electrode thereof being connected to the control electrode of the first switching element, the other main electrode thereof being connected to the other main electrode of said first switching element, and a control electrode thereof being connected to a control electrode of the second switching element.
According to a seventh aspect of the present invention, in the initial current limiting circuit of the sixth aspect, the drive circuit further comprises a second output resistor interposed between the control electrode of the first switching element and the one main electrode of the third switching element.
According to an eighth aspect of the present invention, in the initial current limiting circuit of the fourth aspect, the drive circuit comprises: a second switching element, one main electrode thereof being connected to a source line; a third switching element, one main electrode thereof being connected to other main electrode of the second switching element, other main electrode thereof being connected to other main electrode of the first switching element, and a control electrode thereof being connected to a control electrode of the second switching element; a first output resistor, one end thereof being connected to a connection between the second switching element and the third switching element, and other end thereof being connected to the control electrode of the first switching element; and a second diode connected in parallel to the first output resistor so that a forward current thereof drives the first switching element to turn OFF.
According to a ninth aspect of the present invention, in the initial current limiting circuit of the fourth aspect, the drive circuit comprises: a second resistor, one end thereof being connected to a source line; a first output resistor, one end thereof being connected to other end of the second resistor, other end thereof being connected to the control electrode of the first switching element; a third switching element, one main electrode thereof being connected to the other end of the second resistor, other main electrode thereof being connected to other main electrode of the first switching element; and a second diode connected in parallel to the first output resistor so that a forward current thereof drives the first switching element to turn OFF.
According to a tenth aspect of the present invention, in the initial current limiting circuit of the first aspect, the inrush current limiting circuit further comprises: a switch connected in series to the first capacitor and the first switching element.
According to an eleventh aspect of the present invention, in the initial current limiting circuit of the tenth aspect, the inrush current limiting circuit further comprises: an initial charge controller unit controlling the switch and the first switching element to turn ON on a basis of a start instruction and controlling the switch and the first switching element to turn OFF on a basis of a stop instruction.
A twelfth aspect of the present invention is directed to a power source device. The power source device comprises: a first capacitor, a first switching element of a charge driven type, one main electrode thereof being connected to one end of the capacitor; a second capacitor, one end thereof being connected to the one main electrode of the first switching element, and other end thereof being connected to a control electrode of the first switching element; a DC power source, one end thereof being connected to other end of the first capacitor, and other end thereof being connected to other main electrode of the first switching element; and a switch interposed into a path of a current flowing in circular through the first capacitor, the first switching element and the DC power source.
A thirteenth aspect of the present invention is directed to a power conversion device. The power conversion device comprises: a first capacitor; a first switching element of a charge driven type, one main electrode thereof being connected to one end of the capacitor; a second capacitor, one end thereof being connected to the one main electrode of the first switching element, and other end thereof being connected to a control electrode of the first switching element; a fourth switching element, one main electrode thereof being connected to the one end of the first capacitor; a fifth switching element, one main electrode thereof being connected to other main electrode of the fourth switching element, and other main electrode thereof being connected to the other end of the first capacitor; a sixth switching element, one main electrode thereof being connected to the one end of the first capacitor; and a seventh switching element, one main electrode thereof being connected to other main electrode of the sixth switching element, and other main electrode thereof being connected to the other end of the first capacitor.
According to a fourteenth aspect of the present invention, in the power conversion device of the thirteenth aspect, the power conversion device further comprises: a controller unit controlling the fourth to seventh switching elements so as to perform a normal operation of the fourth to seventh switching elements.
According to a fifteenth aspect of the present invention, in the power conversion device of the fourteenth aspect, the power conversion device further comprises: a switch connected in series to the first capacitor and the first switching element, wherein the controller unit controls the switch and the first switching element to turn ON on a basis of a start instruction and controls the switch and the first switching element to turn OFF on a basis of a stop instruction.
In a device according to the first aspect of the present invention, the device is used by connecting a DC power source to the other end of the first capacitor and the other end of the first switching element and connecting any one of various loads supplied with a power to the one end and the other end of the first capacitor. A fluctuation in a source voltage supplied to the load is suppressed by the first capacitor, and an inrush current flowing through the first capacitor is limited by the first switching element and the second capacitor.
Further, since the first switching element gradually turns ON due to the second capacitor to thereby limit the inrush current, the power resistor through which a large current flows is removed contrary to the first and second prior arts, and it is possible to set a rated power high contrary to the third prior art. Accordingly, the overall weight and volume of the device can be reduced even under a high rated power. Moreover, since the power resistor is removed, when an external switch is turned ON to supply a source voltage, a high voltage is not applied to the switch. Therefore, the reliability and durability of the switch are not harmed.
In a device according to the second aspect of the present invention, since the first resistor is interposed between the first capacitor and the control electrode of the first switching element in addition to the second capacitor, the first switching element is prevented from oscillating and from prematurely turning ON immediately after the external switch turning ON to supply the source voltage. Further, an initial charging current, which flows through the first and second capacitors immediately after the external switch turning ON, is effectively limited by the first resistor.
In a device according to the third aspect of the present invention, since the first diode is connected in inverse-parallel to the first switching element, the first switching element is protected from a reverse current.
In a device according to the fourth aspect of the present invention, since the drive circuit driving the first switching element is provided, a small signal transmitted as a control signal can easily drive the first switching element having a high rated power.
In a device according to the fifth aspect of the present invention, since the drive current driving the first switching element to turn ON is limited by the first output resistor, the first switching element turns ON more gradually so as to limit the inrush current more effectively.
In a device according to the sixth aspect of the present invention, since the first output resistor is provided, the first switching element turns ON more gradually so as to limit the inrush current more effectively. Further, since the second switching element and the third switching element are connected in series and only one of them turns ON, a leak current can be suppressed, thereby reducing drive power consumption.
In a device according to the seventh aspect of the present invention, since the second output resistor is provided, the drive current driving the first switching element to turn OFF can be set at a proper value.
In a device according to the eighth aspect of the present invention, since the first output resistor is provided, the first switching element turns ON more gradually so as to limit the inrush current more effectively. Further, since the second and third switching elements are connected in series and only one of them turns ON, a leak current can be suppressed, thereby reducing drive power consumption. Moreover, since the second diode is provided, the turn-OFF of the first switching element can be speeded up without affecting the turn-ON thereof.
In a device according to the ninth aspect of the present invention, the first switching element can be turned ON further more gradually by the first output resistor and the second resistor, and thereby the inrush current can be limited more effectively. Further, since both drive-ON and drive-OFF of the first switching element are performed by single switching element, the overall weight and volume of the device can further be reduced. Moreover, since the second diode is provided, the turn-OFF of the first switching element can be speeded up without affecting the turn-ON thereof
In a device according to the tenth aspect of the present invention, since the switch is provided, the supply of the source voltage can be started and stopped without preparing an external switch. Further, since the power resistor through which a large current flows is removed contrary to the first and second prior arts, a high voltage is not applied to the switch when the switch turns ON. Therefore, the reliability and durability of the switch are improved.
In a device according to the eleventh aspect of the present invention, since the initial charge controller unit is provided, the switch and the first switching element automatically operates only by giving the start and stop instructions.
In a device according to the twelfth aspect of the present invention, since the device according to the first aspect of the present invention is provided as an inrush current limiting circuit, such a power source device is implemented as to limit the inrush current with high reliability and durability, reducing the overall weight and volume of the device and coping with a high rated power.
In a device according to the thirteenth aspect of the present invention, since the device according to the first aspect of the present invention is provided as an inrush current limiting circuit, such a power conversion device is implemented as to limit the inrush current with high reliability and durability, reducing the overall weight and volume of the device and coping with a high rated power.
In a device according to the fourteenth aspect of the present invention, since the controller unit is provided, signals controlling the fourth to seventh switching elements are not needed to be input from the exterior to perform the normal operation.
In a device according to the fifteenth aspect of the present invention, since the controller unit controls the switch and the first switching element on the basis of the start and stop instructions, the device can easily be handled.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.